Aerodynamics Flashcards
What happens to supersonic airflow when passing through convergent and divergent ducts?
The opposite to sonic flow (convergent: velocity decreases, pressure increases, divergent is opposite)
Sonic flow refers to
Speed AT the speed of sound
Supersonic flow refers to
Speed ABOVE the speed of sound
What effect will an increase in temperature have on the speed of sound
Speed of sound will increase
What is Mcrit?
Critical mach number: The speed where the first evidence of local sonic flow.
When do shockwaves start forming on the wing?
ABOVE Mcrit
What is Mcdr?
Mach critical drag rise number: Drag rises significantly as speeds just above Mcrit
What is the force divergence mach number?
The speed above Mcrit which causes significant changes to the Centre of Pressure.
What is Mdet?
The Mach detachment number. The speed AT which the bow wave attaches to the leading edge (all local flow is supersonic).
What are the mach speeds for the start and end of the transonic speed range?
Mcrit is the start, Mdet is the end.
What is Mmo
Maximum operating mach number, pretty much Vne.
Describe the characteristics of a normal shockwave
sometimes called a vertical shockwave, airflow through the shockwave reduces in velocity to subsonic flow, causing pressure and density to increase. Total pressure after the shockwave reduces.
Describe the characteristics of oblique shockwaves.
Form around compressive corners. Airflow velocity is reduced however REMAINS supersonic, and also experiences a change in velocity and. There is a rise in static pressure and density, and total pressure reduces.
Describe the characteristics of an expansion wave
Forms around expansive corners. Flow direction changes, velocity increases, and static pressure and temperature decrease. Airflow remains supersonic.
What mach speeds are usually considered subsonic?
0-0.75
What mach speeds are usually considered transonic
0.75-1.2
Describe the bow wave during transonic speeds
Forms ahead of the aircraft but is not yet attached to the leading edge
What mach speeds are usually considered supersonic?
1.2-5 (above 5 is hypersonic)
What is Mach tuck?
A tendency for a nose pitch down due to the CP moving rearwards at supersonic speeds.
The main advantages of sweepback are
delayed compressibility effects, increased Mcrit and Mcdr, increased force divergence mach number, Improved lateral stability and directional stability.
The disadvantages of sweepback are
Tendency to stall tip first (strong spanwise flow), reduced Cl max, reduced trailing edge device effectiveness, excessive lateral stability which may lead to dutch roll, high induced drag at high angles of attach, structural complexity.
What is dutch roll?
Spiral stability: result of lateral stability overpowering directional stability. Effect is worsened with altitude, where directional stability reduces, but lateral stability remains the same (no idea why..)
What are the two types of yaw dampers?
Parallel, which move the pilots rudders and therefore must be switched off for takeoff and landing, and series, which is independent and can therefore operate continiously.
What effect does a sweepback wing have on stalling angle of attack?
Increases
to delay seperation, would you prefer laminar or turbulent flow?
Turbulent (no not a typo), because it has higher velocities. This is why vortex generators are effective.
At what speed does a stick shaker usually engage?
1.05 Vs
Following an engine failure in a MEA, you use rudder to counteract the yaw, you expect the balance ball to be
centred
Following an engine failure in a MEA, you use aileron to counteract the yaw, you expect the balance ball to be
fully deflected towards to live engine
Following an engine failure in a MEA, you use rudder and aileron (5° AoB) to counteract the yaw, you expect the balance ball to be
half a ball out
What determines the critical engine?
The engine that results in the most yaw following an engine failure.
For the critical engine, would the down-going propeller be on the inboard or outboard section
inboard
Does a forward CG decrease or increase VMC
decreases (more authority at lower speeds)
What is the effect of gear down on VMC speed?
Increases (due to more drag)
What is the effect of bank angle on VMC?
A small amount of bank (10°) wull reduce VMC when compared to wings level. Due to less rudder needed due to help from ailerons
What is the effect of a windmilling propeller compared to a feathered propeller on VMC speed?
Significantly increase VMC. Really the worst case scenario.
What is the effect of ground effect on VMC?
Reduces VMC speed due to less drag
What is the effect of a change in weight with VMC?
Speed remains the same (approximately)
Vr must not be less than
V1, and 1.05Vmca
V2 must not be less than
1.1Vmca, and 1.2Vs
What will the effect of rotating 5kts early (1 inop) or 10kts early have on TODR?
Not alter TODR
V2 is the
Takeoff Safety Speed
Usual causes of tailstrike are the result of
Early rotation or rapid rotation (more than 3°/sec).
When climbing at a constant mach number, the Calibrated Airspeed (CAS) will
Decrease
When climbing at a constant mach number, the TAS will
decrease if the temperature decreases
The difference between leading edge slats and leading edge flaps is that
slats re-energise the boundary layer and flaps after the effective camber of the leading edge
As an aicraft climbs above FL300, the IAS and EAS stall speeds will
both increase (though IAS more than EAS)
An aircraft accelerates from subsonic speeds to mach 1.0 to supersonic speeds, the coefficient of drag will
increase then decrease
In a constant mach number climb in the isothermal layer above the tropopause, the TAS will
remain constant due to the constant temperature
In a constant mach number climb in the isothermal layer above the tropopause, the IAS will
decrease due to decreasing pressure
Mcrit is
The speed at which sonic flow occurs, and therefore the speed above which supersonic flow occurs.
If the aircraft exceeds Mcrit and no mach trimmer is fitted, the aircraft will
pitch down
For high subsonic cruise speeds, the reason it is is best to delay the formation of shockwaves is
to keep the drag penalty to a minimum
Fast transport category aircraft may have a supercritical (flat) wing section, this is to
reduce the top wing surface acceleration.
Aileron control reversal can be caused by
the wing twisting about its lateral axis due to high aerodynamic loads at high speed
Exceeding Mcrit results in a nose down pitch, this is because
the CP moves rearward
An aircraft is slowly accelerating through its Mcrit to transonic cruise, the coefficient of lift
will increase and then decrease
A mach trimmer is designed to
ensure that with an increasing speed, the aircraft will tend to pitch up
At transonic flow, the reason that some aircraft use spoilers for roll control even though they have ailerons fitted is
to overcome the loss of aileron effectiveness due to flow separation ahead of the aileron
Wing sweep results in an increase in the stalling angle of attack, this is because
swept wing Cl is less than straight wing Cl for any given body angle
Wing sweep is an important feature for high speed flight because
it delays the drag effects of shockwaves
On swept wing aircraft, nose pitch up at the stall is caused by
wing tip stall and the effective CP moving forwards
Jet aircraft ahieve maximum RoC at
maximum total POWER over WEIGHT
You want your jet aircraft to stay airborne as long as possible, you would achieve this by flying at an altitude where
engine design RPM would coincide with Vimd
To achieve maximum angle of climb, you would climb at
maximum thrust over drag (Vimd)
The Centre of pressure should ideally have minimal movement with variations in speed because
large trim changes would be needed otherwise
When does the stick pusher activate
After the stall
Jet overspeed warnings occur
slightly ABOVE Mmo and Vmo
Mfs (free stream) is
The actual mach number of air passing the aircraft
Airflow through a shockwave will experience
An increase in temperature, density, and pressure, but a decrease in velocity.
A typical Jet SAR figure is expressed as
100anm/1000kg
To achieve maximum range at any given flight level, you would fly at
1.32 Vimd
Optimum range is achieved by flying at
a cruise climb, 1.32Vimd, and design RPM
Climb IAS decreases with increased altitude because
Maximum excess power occurs at a lower IAS at high altitude (the graph moves up and to the left)
A jet climbs at a constant mach number after a certain altitude is reached because
this equates with the required reducing IASand to prevent mach overspeed
If required to cruise at a level with a significant headwind when maximum range is required, would you fly faster or slower than 1.32 Vimd?
Faster, as you need to achieve the best GS/drag ratio
As airflow passes through a normal (vertical) shockwave, what happens to velocity and direction?
Velocity changes from supersonic to subsonic, with no change in direction
What is the purpose of slotted flaps?
Increase the camber of the wing and preserve energy in the boundary layer
What is the purpose of fowler flaps?
Increase the camber and chord of the wing
Flap extension at high altitude is not permitted on most aircraft because
flap extension speed may be below the stalling speed at high altitude
Are trailing edge devices more effective on swept back wing or a straight wing?
straight wing.
An internal flexible seal attached to the leading edge of a control surface is designed to…
assist control load balancing
The Vmcg speed is lower than the Vmca speed because…
The wheels are on the ground and provide more control
When flying at a very high altitude in a swept wing jet transport category aircraft, vibration felt through the airframe during manouvres may be caused by
either high or low speed buffets
True or false, cruising at a higher FL will always increase SAR?
False
True or false, Maximum endurance is achieved at the best lift/drag ratio
True
True or false, At all altitudes, mach number is the ratio of TAS to the local speed of sound AT MSL
false (would be true if not for MSL)
True or false, any head or tailwind component will reduce the distance to a PNR?
True
True or false, the mach number required for maximum range cruise is independent of hw or tw
false
True or false, low SFC is best achieved at low altitude and low engine RPM?
Flase
True or false, the angle of attack required for maximum range cruise does not vary with aircraft gross weight
true
True or false, maximum static thrust increases with increasing ambient temperature
false
True or false, during constant altitude cruise, a reduction in aircraft GW will require a reduction in wing angle of attack to maintain the optimum lift coefficient for subsonic cruise?
false. speed will change, AoA remains the same.
True or false, Mcrit is that free stream mach number below which all local airflow is subsonic
True
True or false, high propulsive efficiency is typified by low mass flow and maximum possible velocity change?
False
True or false, engine SFC is directly proportional to thrust
False
True or false, for cruise at a constant mach number and constant altitude, buffet boundary margins increase with increasing gross weight
False
True or false, wing sweep back increases Mcrit
True
True or false, at a constant cruise level, the long range cruise mach number decreases with decreasing gross weight
true
True or false, the angle of attack for best endurance at a constant altitude decreases with a decreasing gross weight
false (LRC aim is to fly at constant angle of attack)
True or false, the minimum thrust and minimum angle of descent is obtained at L/D max.
True
True or false, at a constant IAS and FL, mach number is not affected by changes in air temperature
True
True or false, the long range cruise mach number increases with a decreasing gross weight
false
True or false, below the tropopause, a constant mach number cruise climb would produce a constant IAS
False
True or false, both maximum endurance and maximum climb angle performance occur at L/D max.
True (Vimd)
To calculate Vmca speeds in the AFM, is a rearward or forward CoG assumed?
Rearward
The critical engine is that engine with…
the down-going blade closest to the fuselage
In relation to TAT, OAT is always
less than or equal to TAT
Climbing at IAS just below Vmo will see Mmo….
eventually exceeded
In relation to CAS, EAS is always
less than or equal to
ICE-T, Its a pretty cool drink
IAS, CAS, EAS, TAS, Instrument and Position error for IAS to CAS, Compressibility for CAS to EAS, and Density error for EAS to TAS.
What happens to TAS when climbing at a fixed mach number?
TAS reduced (speed of sound reduces as temp reduces, therefore TAS reduces to maintain mach).
